This invention relates to motor generator equipment for transforming mechanical power into regulated power using on electronic power converter and, in particular, a half-bridge inverter for providing a generally sinusoidal output and a stabilized neutral node suitable for use in lightweight portable generator systems, such as, for example, the type disclosed in commonly assigned U.S. Pat. No. 5,625,276 to Scott et al., filed Jan. 9, 1995, and various commonly assigned, co-pending applications.
Commonly assigned and co-pending application U.S. Ser. No. 08/752,230 now U.S. Pat. No. 5,886,504, by Scott, et al. filed on Nov. 19, 1996 entitled "Throttle Controlled Generator", describes among other things, a lightweight, compact power conversion system employing an engine and an alternator capable of providing a regulated voltage regardless of engine speed and load current fluctuations. One of the embodiments described there provides a relatively high voltage, low current AC output suitable for powering lighting and appliances, a relatively high current output suitable for battery charging and starting vehicles, and an output suitable for arc welding. That system includes an alternator having multiple stator windings, one or more switching rectifier circuits, an inverter, and a controller. In general, alternator windings produce a signal (typically multi-phase) that is applied to a switching rectifier suitably configured as a plurality of switching rectifier circuits, to generate one or more uni-polar voltages (generally, "DC rail voltages," or "DC rails"). One or more DC rail voltages may be provided to the inverter to generate an output voltage having a sinusoidal waveform. The controller (e.g., a microprocessor based circuit) selectively activates and deactivates the various windings, provides control signals to the switching rectifier circuits to maintain the desired DC rail voltages, and operates the inverter to maintain output voltages under varying load conditions.
Commonly assigned and co-pending applications by Scott et al., U.S. Ser. No. 08/370,577 now U.S. Pat. No. 5,625,276, filed Jan. 9, 1995, Ser. No. 08/695,558 now U.S. Pat. No. 5,900,722, filed Aug. 12, 1996, and Ser. No. 08/752,230 now U.S. Pat. No. 5,886,504, filed Nov. 19, 1996 describe various inverters. In general, the inverters described there include a converter circuit, for example, an H-bridge configuration of switching devices. Each switching device couples one of two DC rail signals to one of two juncture nodes. Each juncture node is coupled to an inverter output terminal. One of the DC rails is considered a common rail, being negative or of lower potential relative to the other DC rail. Each switching device is responsive to a respective drive signal. The magnitude of the voltage between the juncture nodes is controllably varied to create a predetermined waveform at the inverter output terminals (e.g., a simulated sine waveform). Various mechanisms are described there for varying the voltages of the juncture nodes relative to the common rail.
The aforementioned commonly assigned U.S. Pat. No. 5,625,276, issued Apr. 29, 1997, and the commonly assigned U.S. patent application Ser. Nos. 08/752,230, 08/370,577, and 08/695,558, now U.S. Pat. Nos. 5,886,504, 5,625,276, and 5,900,722, are incorporated herein by reference.
In general, half-bridge converter circuits are known. Examples of such half-bridge converters are described in U.S. Pat. No. 5,253,157, issued on Oct. 12, 1993 to Severinski; U.S. Pat. No. 3,775,663, issued Nov. 27, 1973 to Turnbull; U.S. Pat. No. 4,564,895, issued Jan. 14, 1986 to Glennon; U.S. Pat. No. 4,833,584 issued May 23, 1989 to Divan; and U.S. Pat. No. 4,814,962, issued Mar. 21, 1989 to Magalhaes, et al. See also Power Electronics, Converters, Applications, and Design, 2nd Edition, by Mohan et al., John Wiley & Sons, Inc., 1995; and Design of Solid-state Power Supplies, 2nd Edition, by Eugene R. Hnatek, Van Nostrand Reinhold, 1981.
Referring briefly to FIG. 1, a conventional half-bridge inverter 100 typically comprises respective switching devices S104 and S106, capacitors C108 and C110, and a suitable filter F116. Switching devices S104 and S06 (for example, contactors, semiconductor devices, or switching circuits) are connected in series between a relatively positive DC rail RL1, and a relatively negative DC rail RL3, with a juncture node J121 therebetween. Capacitors C108 and C110 are likewise connected in series between DC rails RL1 and RL3, and define a juncture node J122 therebetween. A juncture node (or simply juncture) is any nominal point of electrical connection, for example an output node of a bridge circuit. Anti-parallel diodes are typically included in switching devices S104 and S106 for eliminating reverse voltage breakdown failures of components of the switching devices. Capacitors C108 and C110 are of equal and sufficiently large capacitance to provide current through load Z150 when a circuit through one of the switching devices is completed.
Switching devices S104 and S106 are rendered conductive and non-conductive by drive signals DS104 and DS106 to selectively connect DC rail RL1 or RL3 to juncture J121 and generate a pulse width modulated (PWM) signal at juncture J121 relative to DC rail RL3. The PWM signal has a constant frequency and constant amplitude when "on"; however, the duty cycle varies suitably from one period to the next according to predetermined samples of a symmetric sinusoidal waveform. A PWM signal is conventionally averaged by a low-pass filter to recover the sinusoidal waveform. Such a filter averages the "on" and "off" portions of each period. For example, filter F116 is a single-stage passive LC network having inputs connected to junctures J121 and J122 and having outputs connected to output terminals T118 and T119. After filtering by filter F116, the inverter output signal V.sub.out at terminals T118 and T119 is provided across load Z150 with a relatively accurate sinusoidal waveform.
Capacitors C108 and C110 conduct currents of the PWM signal to rails RL1 and RL3. Consequently, the voltage at juncture J122, relative to common rail RL3, is biased about a midpoint reference voltage of approximately one half of the potential between DC rails RL1 and RL3. The voltage at juncture J121 referenced to juncture J122 has a PWM waveform with pulses above and below the mid-point voltage. The filtered voltage appearing across load Z150 has a symmetric sinusoidal waveform without the bias.
In half-bridge inverter applications in the prior art, there remain problems associated with maintaining both the rectified DC level V.sub.in across terminals T101 and T102 and maintaining symmetry of the voltage waveform at juncture J122 about the mid-point reference voltage. As a consequence, the magnitude of the output voltage V.sub.out becomes unstable and generally decreases in magnitude. Further the signed NEUT terminal T119 varies in absolute voltage and is no longer reliable as a stable neutral node.
Unequal time constants associated with capacitors C108 and C110 in combination with response timing errors in switching devices S104 and S106 lead to voltage drift at juncture J122 from the mid-point reference voltage. Analysis has shown that drift from the mid-point reference voltage can arise from manufacturing tolerances, can increase with aging (of capacitors, switching devices, and loads), and can change with changes in load Z150. At typical line operating frequencies (e.g., 50, 60, and 400 Hz) and typical load currents, the capacitance of capacitors C108 and C110 is required to be particularly large, for example on the order of several Farads for an output of only a few Amperes at 60 Hz. Capacitors of this magnitude are prohibitively expensive for many applications and unwieldy in physical size and weight for many portable applications. Non-uniform aging is more likely with larger capacitance capacitors, exacerbating voltage drift.
It is, in general, known that the voltage at juncture J122 can be stabilized to reduce drift by employing a further set of switching devices that selectively couple an inductor across one or the other of the capacitors. However, increased circuit complexity adversely affects product prices and increases maintenance and repair costs.
It is desirable that an inverter provide substantial current at a stable output voltage and a designated output frequency economically at operating switching frequencies compatible with available switching devices. Voltage drift adversely affects the magnitude of the output voltage, consequently disturbs current through load Z150, and generally degrades operating functions of the load. Voltage drift can also lead to conditions that are unsafe for personnel and equipment.